Log Message:
lintpkgsrc: fix Perl warning when parsing variable expressions

When parse-guessing a package Makefile, lintpkgsrc tries to evaluate
variable expressions such as ${VAR:S,from,to,} by passing them through
Perl's eval function.

In a variable expression of the form ${VAR:S,@exec@,${exec},}, this
produced Perl warnings due to the unescaped '@':

	Possible unintended interpolation of @exec in string at
	(eval 63841) line 1.

As a quick fix, skip ':S' modifiers that contain the character '@' for
now.  A proper fix will follow.